1. Field of the Invention
The present invention relates to a developing apparatus which visualizes an electrostatic latent image formed on an electrostatic latent image bearing member by use of toner serving as a coloring agent, specifically to a developing apparatus which utilizes a developer composed of one component as toner.
2. Description of the Related Art
An image forming apparatus such as copying machines and printers which adopts an electrophotographic system is equipped with a developing apparatus which forms an electrostatic latent image on a surface of a photoconductor serving as an image bearing member, supplies a developer such as toner serving as a coloring agent to the photoconductor for the purpose of visualizing the electrostatic latent image, and causes selective adhesion of the toner.
By the developing apparatus, an electrostatic latent image formed on the photoconductor is developed and the developed toner image is transferred onto a transfer material such as a sheet. Thereafter part of the toner which was not put to the transfer remains on the surface of the photoconductor. This remaining unnecessary toner is removed from the surface of the photoconductor so that formation of an image is subsequently carried out. For this purpose, a cleaning device for removing the toner remaining on the surface of the photoconductor after the transfer is disposed, which cleaning device includes a container for receiving the unnecessary toner removed by the cleaning device.
In order to meet the need of downsizing an image forming machine which is equipped with such a developing apparatus as described above, a space for disposing processing means for forming an image around the photoconductor is narrowed, with the result that downsizing the developing apparatus is also strongly requested.
In specific, the developing apparatus is equipped with a developing roller adopting a magnetic brush system which conveys a dual-component-type developer composed of toner and magnetic carrier to a developing region facing the photoconductor by utilizing a magnetic force, thereby collecting the developer into a developing tank after development. Therefore, in order to stabilize development, it is necessary to replenish consumed toner and control the ratio of toner contained in a developer, that is, the concentration of toner so as to become constant.
In general, in the developing apparatus adopting the aforementioned system, that is, the magnetic brush developing system, the ratio of carrier to a developer is larger than that of the toner and accordingly the developing tank for containing the developer becomes large, whereby the developing apparatus is likely to become large as a whole. In addition, it is necessary to provide an agitating member for making a charging amount of the toner in the developer constant as well as to control the concentration of the toner. In such system, two or more agitating members are provided, which has been a bottleneck for downsizing the developing apparatus.
In response to this, such a developing apparatus is proposed and practically used that conducts development with the use of a one-component-type developer, that is, toner which is a one-component-type developer without carrier. According to such a developing apparatus that uses one-component toner, it is not necessary to control the concentration of toner, it is possible to reduce the size of the developing tank to a considerable degree owing to the inexistence of carrier, and hence it is possible to downsize the developing apparatus. In addition to this, the developing apparatus is excellent in simplicity of maintenance and so on. In short, since there is no need to replace a developer degraded, specifically due to the degradation of carrier, maintenance for the replacement is eliminated.
Further, it is required only to replenish toner, there is no need to detect the concentration of toner, and control for the detection is not required, with the result that control of the developing apparatus is simplified. In specific, as for the developing apparatus using the one-component-type toner, it is required only to replenish toner when necessary.
For instance, as shown in FIG. 1, a developing apparatus 4 is placed so as to face a photoconductor 1 serving as an image bearing member, the developing apparatus 4 visualizing an electrostatic latent image formed on the photoconductor 1. In the developing apparatus 4, a developing roller 41 is disposed in a rotatable manner so as to specifically face an opening portion of a developing tank 40 which contains toner serving as a one-component-type developer. The developing roller 41 is partially exposed to the opening portion of the developing tank 40, and is positioned so as to come into contact with the photoconductor 1, for example. This contact region forms a developing region.
The developing roller 41 carries the one-component toner on the surface thereof and conveys it to the developing region which faces the photoconductor 1. After development, the developing roller conveys and collects toner which was not used for the development into the developing tank 40. In order to once remove the collected toner from the surface of the developing roller 41, a supplying roller 42 is disposed so as to be pressed to the developing roller 41, whereby the toner carried on the surface of the developing roller 41 is scraped off. Then, toner is newly supplied onto the surface of the developing roller 41 by this supplying roller 5.
The one-component toner is supplied by the supplying roller 42 and absorbed onto the surface of the developing roller 41. In order to regulate the absorption amount, a regulating member 43 which is pressed to the surface of the developing roller 41 is disposed. Toner which has passed by the regulating member 43, the amount thereof being regulated to be constant, reaches the developing region which faces and comes into contact with the photoconductor 1 as mentioned above, and selectively adheres in accordance with the electrostatic latent image formed on the surface of the photoconductor 1, whereby development is conducted.
Further, for the purpose of achieving satisfactory development, a developing bias voltage (Va) is supplied to the developing roller 41 in general. This developing bias voltage is set to a voltage value such that the toner adheres to the electrostatic latent image while the toner does not adhere to a background region (a background portion excluding an image) of the photoconductor.
Also in order to charge the one-component toner absorbed to the developing roller with a predetermined polarity and add a predetermined amount of potential thereto, the regulating member 43 which is pressed in the aforementioned manner is disposed on the downstream side in the rotation direction of the developing roller 41, supplying to the regulating member 43 a regulating voltage (Vb) for charging the one-component toner with the predetermined polarity. Thus, as a result of passing by the regulating member, the one-component toner which is kept in a constant amount is conveyed to the developing region while being charged to be at a predetermined potential or higher.
With the above configuration, the one-component-type developer, namely, the toner is attracted onto the developing roller and conveyed to the developing region, whereby the toner adheres onto the electrostatic latent image on the photoconductor. Accordingly, it is realized to prevent the toner from adhering to the background excluding the latent image to conduct satisfactory development.
The developing apparatus of the above configuration makes the developing roller 41 into contact with the photoconductor 1 to conduct development, so that the setting of a resistance value of the developing roller 41 is an important factor for determining development characteristics. In response to this, hitherto a variety of techniques have been proposed. For instance, it is aimed to conduct development in a preferable manner by using a developing roller of high resistance.
However, even when a developing roller of high resistance is used, the variance of resistance value with temperature and humidity is large and development characteristics widely vary due to the variation of resistance value, so that a problem of large variation in image density occurs. In addition, a development ghost is likely to be formed due to electrical charges accumulated on the surface of a high resistance layer of the developing roller is likely to be formed.
In order to solve these problems are implemented a variety of development-assisting means, however, such means lead to an increase in total cost.
Further, there is also proposed an idea of solving the aforementioned problems by using a developing roller of low resistance. However, according to this method, an electrical breakdown of a toner layer and an overcurrent are likely to occur because of using the developing roller of low-resistance, and therefore it is difficult to apply this idea to a practical use as it is. Then, Japanese Unexamined Patent Publication JP-A-2-93671(1990) and Japanese Unexamined Patent Publication JP-A 3-87759(1991) propose a method of stabilizing development characteristics by connecting a protection resistor which is a high resistor of 1 M.OMEGA.-100 M.OMEGA. to the development roller of low resistance.
As mentioned above, recently, downsizing of copying machines and printers is required with the trend toward faster operations thereof, and therefore a technique for ensuring a developing performance in the developing region of the developing apparatus is desired as well as downsizing of the developing apparatus itself.
That is to say, in order to meet the faster image forming apparatuses, an idea for efficiently conveying a developer in the developing apparatus is made, and in order to also downsize the developing apparatus so as to meet downsizing of the image forming apparatuses, a developing apparatus using one-component toner is required.
In such a developing apparatus using one-component toner, developing characteristics are kept constant by stabilizing the resistance value of the developing roller, whereby excellent development is realized. The developing roller is made to be of low-resistance, to which a high resistance device is connected, however, such procedure is still insufficient.
In other words, due to a problem concerning the one-component toner itself, an overcurrent occurs when the one-component toner comes into contact with the photoconductor. In short, it is not sufficient to merely supply a predetermined voltage to the regulating member 43 for forming uniformly charged toner onto the developing roller in a uniform layer-thickness, and therefore, a considerable voltage is supplied to the supplying roller 42 which supplies toner to the developing roller 41. For this reason, in the case where the developing roller 41 of low resistance is used, an electrical breakdown of a toner layer and an overcurrent occur with the result that a preferable developing characteristic cannot be obtained.
The occurrence of overcurrent can be prevented by setting the internal resistance of toner itself to be high. However, an external additive is added to the toner in order to avoid various problems such as fluidization. Therefore, the surface resistance and contact resistance of the toner often become material factors owing to the influence of the external additive, so that it is impossible in fact to set the resistance value of the toner which is in the form of a thin layer at a high level or to keep it at a constant value or more only by controlling the internal resistance value of the toner.